The present invention relates to sensors, and in particular to the compensation for vibration-induced error affecting the operation of such sensors.
Inertial sensors, such as gyroscopes and accelerometers, are often adversely affected by vibrations. They may be very sensitive to such vibrations. In fact, vibration sensitivity can often be a dominant performance limiting error source. Many inertial sensors rely on micro-electromechanical sensor (MEMS) devices which utilize vibratory mechanisms for rate/acceleration sensing. These types of sensors have shown evidence of significant vibration sensitivity problems.
Inertial sensors are often used in environments that inherently subject the sensors to significant vibrations. These include use in aircraft and weapons, where the power plants and environment serve as significant sources of vibration. For example, a commonly used resonant beam accelerometer that is often viewed as a one milli-g accuracy sensor can exhibit bias shifts on the order of several milli-g""s under typical operational vibration environments.
For inertial sensors which exhibit vibration sensitivity, the most common effect is a slowly varying low frequency error component which changes as a function of the applied vibration spectrum. In some applications, such as a global positioning system (GPS) aided inertial system, the effect of the vibration induced bias shift can be trimmed or estimated such as by use of a Kalman filter. However, when the vibration level or spectrum shape changes significantly, the vibration-induced error also changes, diminishing the value of any previously obtained calibration.
In the case of an inertially guided munition launched from an aircraft equipped with a high quality inertial navigation system (INS), an in-flight transfer alignment and calibration procedure is usually performed prior to release of the guided munition. During this period, velocity differences (or related quantities) between the aircraft INS and the weapon INS are processed by a Kalman filter to initialize the attitude and heading of the weapon INS, and to estimate some of its inertial sensor errors, typically including the gyro and accelerometer bias errors. The vibration spectrum present at the weapon inertial measurement unit is strongly driven by its captive-carry environment, such as a wing-store station or weapon bay. During the captive carry, the weapon""s inertial sensor bias errors will be affected by the vibration environment. However, after release of the weapon, a significantly different vibration environment will exist, leading to gyro and accelerometer bias shifts due to a change in the vibration-induced errors. Depending on its magnitude, this change can negate any benefit of the pre-launch weapon inertial measurement unit calibration. There is a need to minimize the detrimental effects of such changes in vibration environment.
Other factors can also cause such bias shifts. Anything that causes a change in the vibration environment of a sensor can result in a bias shift. Changes in speed of any vehicle having such sensors can result in different vibration environments due to aerodynamic differences and also due to mechanically based resonant frequency changes. This can by observed by any passenger in a car, especially older cars, as it approaches speeds that are higher than intended, or equipment of the vehicle changes through damage. Vibration environments can also change as engines are operated at different revolutions per minute, or depending on the overall load condition of the vehicle.
There is a need for a better mechanism to compensate for vibration-induced errors affecting inertial sensors. There is a further need to compensate for changes in vibration environments that affect such sensors. There is a need for such compensation to be performed both accurately and quickly.
A factory calibration procedure is performed by subjecting a sensor to suitable vibration stimulus, thereby exposing sensitivity coefficients relating vibration power levels at various frequencies to the bias-like error of the sensor. In the real-time application of the sensor, accelerometer samples are collected and used to characterize the actual vibration environment, specifically, the amount of vibration energy in various portions of the frequency spectrum. Predicted sensor vibration-induced error is calculated based on the measured vibration spectrum and the previously-obtained factory calibration coefficients. The predicted vibration-induced error is then applied as a compensation to the sensor output.
In one embodiment, high rate vibration measurement samples from the accelerometers are provided as the input to a power spectral density estimator. The vibration power spectrum estimates are then combined with previously obtained calibration coefficients to generate the real time vibration-induced bias correction. Vibration bias sensitivity coefficients are estimated in a controlled environment to ensure their accuracy to compensate the sensor in real-time applications.
In a further embodiment the high data rate vibration samples are obtained using acceleration measurement devices such as accelerometers mounted to a rigid sensor assembly shared with the sensor to be compensated. Vibration bias estimates are then combined with outputs from both a plurality (typically three or more) of gyros and a plurality of accelerometers, which combined form an inertial measurement unit.
In yet a further embodiment, a digital signal processor is used to perform calculations to obtain the vibration power spectral density and then provide the sensor vibration-induced error estimates in real time at a high rate. Still further embodiments utilize software to perform the calculations, or further combinations of hardware, software and firmware.